Mesoscopic Structures in Gel and their Evolution on Dehydration

点击翻译按钮获取中文摘要

A gel is a system composed of a cross-linked polymer network and solvent and is therefore a multicomponent system. When the constituents are highly compatible with each other, the polymer network spreads out into the solvent to maximize the entropy of mixing. Conversely, the network shrinks to minimize the surface energy when the constituents are incompatible. For example, with increasing temperature, N-isopropylacrylamide (NIPA) gel exhibits a volume reduction at 36℃. This phenomenon is well-known as the volume phase transition. This project explores the property changes induced in a gel by dehydration and another mechanism to produce a shrunken gel. In addition to overall decrease in volume, other properties show drastic changes during dehydration and the gel exhibits a glass-like behavior. This has been confirmed with viscoelastic measurements, via the observation of a low-frequency peak in Raman scattering, and also by inelastic neutron scattering experiments. Furthermore, a small-angle X-ray scattering study reveals a distinct mesoscopic structure in a heteropolymer hydrogel, which exhibits microscopic separation upon dehydration. The solvent affinity of the NIPA group does not change appreciably because the dehydration was conducted at room temperature. Therefore, we anticipate that microscopic separation of the heteropolymer gels is a general phenomenon, which is nor restricted to the volume phase transition, and requires a balanced set of gel constituents with competing solvent affinities.